Proper pacing when you go from second in the line to the front is something of an acquired skill. It takes a little practice to do it well. Riders brand new to the experience will sometimes ride more slowly, since they don't always know that they need to compensate for the new wind resistance hit. Ones who think they know what they're doing will overcompensate, assuming they need to work a lot harder at the front than they were when they were drafting, and without realizing it, they've bumped the line's speed up a bit. There's a balance you have to maintain there, and if you find your train is falling off your pace, you're failing at your task.
The part of this I have the hardest time with is slight rises in the road. I tend to try to maintain the same speed over them, only because it's relatively easy to do, and I know what the back of a line feels like when the leaders don't do that. That's when you get a lot of braking and shouts of "slowing!" I try to avoid that sort of thing when I'm at the front. Sometimes it works, sometimes it doesn't. It just depends on the kind of riders you're with and whether they feel like following your lead.